For economic reasons, mainly contactless measuring methods are employed for inspecting glossy or high-gloss surfaces such as those of alloy wheels, for example, for surface defects such as dents, bumps, drops or bumps of lacquer (paint), scratches or grooves, for example. In the functional field of industry, surface defects are typically sensed and detected by means of optical capturing methods.
With sufficiently cooperative, i.e., diffusely scattering, surfaces, shape measurement may also be affected by means of optical scanning, for example by means of optical triangulation methods. By means of optical triangulation methods such as a laser light-slit method, for example, surfaces comprising a sufficiently large diffuse scattering contribution can be sensed and measured. Said methods are not suitable in most cases for surfaces having a large or a very large amount of gloss. In addition, they exhibit a limited spatial resolution and are not, or hardly, sensitive to small local differences in inclination of the surfaces as occur on above-mentioned surface defects of alloy wheels, for example.
Quality control with regard to surface defects of specular surfaces such as on alloy rims, for example lacquer defects, casting defects, mechanical damages or inclusions within the material, so far has mostly been performed by means of a visual examination on the part of human examiners for lack of alternative test procedures. Said visual examination may be very time-consuming and/or costly. In addition, such visual examination is subjective and may not be reproducible and/or quantifiable in a reliable manner. Fatigue or fluctuations in the ability to concentrate on the part of the human examiner may give rise to the risk of defect slippage.
Said surface defects are visible to the human observer due to a local variation of the reflected ambient light. Small or very small lacquer defects which have a small height extension, for example within the micrometer range, may possibly not be reliably sensed by means of the laser light-slit method. Deflectometry methods are suited to sense especially highly reflecting surfaces with regard to their shapes, the local angles of inclination and surface defects, and/or to measure the shape of the surface. Typically, a sequence of bright and dark strips is produced on an extensive light source, e.g., a fluorescent screen, and sequentially mapped (projected, imaged) in reflection via the specular surface by means of an area scan camera. The course of the striped pattern within the sequentially taken cameral pictures depends on the local and global shape, i.e., on the position and inclination, of the surface and may be evaluated automatically, i.e., by means of a computer program, from the sequence of pictures taken. On the condition that the arrangement of the camera, the object (surface) and the light source is calibrated, the three-dimensional shape of the surface may be reconstructed in the course of the evaluation. An evaluation of local shape deviations may possibly also be achieved without a fully calibrated setup and/or reconstruction algorithm.
In the deflectometry method, a sequence of pictures may be taken, via an area scan camera, for each surface area to be captured; the relative position between the camera, the surface and the fluorescent screen normally may not change during a capturing process, i.e., relative motion may not take place. In addition, in this method the camera pictures that have been captured and are to be evaluated may be transmitted, for data processing, to an evaluating unit, for example an evaluation calculator. The data transmission rate for transmitting two-dimensional camera pictures to a computer is currently limited, for technical reasons, to a maximum of 100 pictures/s, so that the object is typically moved discontinuously and so that a reduced testing throughput is achieved. The useful step-wise positioning of the object, or of the surface, in relation to the metrological arrangement of the camera and the fluorescent or projection screen (or vice versa, of the metrological arrangement in relation to the object) may additionally involve a large amount of effort, i.e., may be costly and/or relatively slow, with regard to the mechanical components that may be used such as with regard to the axes of displacement that are to be realized.
For automatically detecting surface defects that are very small in terms of their surface areas or heights, a correspondingly high pixel resolution of the camera and a correspondingly finely resolved striped pattern may be used. On account of the sampling theorem it may be useful for a lateral extension of the pattern, i.e. a stripe width of the extension, to correspond to or fall below the extension of the defects to be detected.
DE 102004007828 A1 describes an optical method of sensing surfaces of three-dimensional bodies wherein the surface, the illuminating means and the camera may be made to have at least one defined geometric relation, for the period of the respective picture taking, by means of movement means, so as to achieve a constant capturing quality for each area to be inspected and a high recognition rate for defects. During the exposure and/or picture-taking time of the camera, relative motion between the camera, the surface and the illuminating means may not take place. This means that the entire surface may be sensed step-by-step, i.e. one surface region after the other, whereby a reduced testing throughput is achieved since the arrangement of the surface, the camera and the illumination may be repositioned in each case between the individual picture taking operations.